1. Technical Field
The present invention is directed to an in vitro aqueous composition of the drug, FK506 having enhanced stability. The invention utilizes blood cells or fragments of blood cells to stabilize the drug in an aqueous matrix, preferably an aqueous matrix also containing an alkali halide.
2. Background
FK506 is an immunosuppressant useful for the treatment of rejection following transplant surgery, graft versus host disease and autoimmune diseases in humans. FK506 corresponds to the following structural formula, (I): ##STR1## FK506 is a macrolide antibiotic isolated from the fungus Streptomyces tsukubaensis by the Fujisawa Pharmaceutical Company of Japan. Cyclosporine, another immunosuppressant (but having a totally different structure from FK506 ), has also been used to control rejection. During cyclosporine therapy, monitoring the blood concentration of cyclosporine is an important aspect of clinical care. Accordingly, it is expected that monitoring blood concentrations of FK506 will be important for patients receiving this drug.
To accurately and precisely measure blood concentrations of FK506, an appropriate analytical method must be available. Typically, many analytical methods require utilization of a standard composition of the analyte to be measured, often as an essentially aqueous composition. However, FK506 tends to degrade rapidly in aqueous matrices. Accordingly, there is a need for stable aqueous compositions of FK506 for utilization, for example, as aqueous standards for diagnostic assays for FK506.
EP 0 293 892 A2 describes an ELISA methodology to measure FK506 comprised of 1) an ELISA plate coated with anti-FK506 antibodies, 2) an FK506-horseradish peroxidase conjugate which competes with free FK506 and acts as a signal generating reagent and 3) an appropriate substrate for the peroxidase. While the ELISA assay is effective, it is subject to improvement. Automated or semi-automated immunological technologies are typically both more precise and accurate and have greater throughput than ELISA formats. The Abbott IMx.RTM. analyzer is one such system which can run, for example, microparticle capture enzyme immunoassays (MEIAs). A number of serum-based assays have been developed for the IMx.RTM. but thus far no whole blood tests have been developed which are compatible with the MEIA format. Moreover, the need for providing an aqueous composition of FK506 of enhanced stability has been a challenge in the development of a diagnostic assay for FK506 which uses whole blood. A semi-automated whole blood assay for FK506 is described herein which provides a much needed and reliable means of measuring blood FK506 concentrations in patient samples and which utilizes, as a standard, a stabilized aqueous matrix containing FK506.